Turning to FIG. 1, an example of a conventional pipelined ADC 100 can be seen. ADC 100 generally comprises a pipeline (which receives an analog input signal AIN) that provides digital signals to a digital output circuit 106 so that a digital output signal DOUT can be generated. The pipeline is generally comprised of a buffer 108, output ADC 104, and ADC stages 102-1 to 102-N (which are generally arranged in a sequence). Each of the ADC stages 102-1 to 102-N generally comprises a track-and-hold (T/H) circuit 112, a sub-ADC 118, DAC 120, and a residue amplifier 122. In operation, as shown in this example, T/H circuit 112, for each ADC stage 102-1 to 102-N, receives an input signal (i.e., signal AIN or the residue from the previous stage) and samples the signal based on clock signal CLK. Sub-ADC 118 (which also uses the clock signal CLK) converts the sample to a digital signal, which is provided to digital output circuit 106 and DAC 120. Residue amplifier 122 then amplifies the difference between the sampled signal (from T/H circuit 112) and the output from DAC 120, which is the residue signal or residue of the stage. The final ADC stage 102-N of the sequence then provides its residue to output ADC 104, which provides a digital signal to digital output circuit 106.
Turning now to FIG. 2, a more detailed example of DAC 120 can be seen. Typically, sub-ADC 118 is a coarse ADC having 2n levels, which can provide a control word to the DAC 120. This control word can be thermometer coded with 2n levels and can be used to control DAC switches 202-1 to 202-R (where each switch 202-1 to 202-R can generate a “+1” or “−1”). To accomplish this, control signals (which are generally derived from the control word) can be provided to transistors QU1 to QUR and QD1 to QDR so as to enable current to be sourced through the “+1” or “−1” paths (through the respective current source 204-1 to 204-R). A problem, however, is that, regardless of the code for DAC 120, the noise from the current sources 204-1 to 204-R can be observed at the output of residue amplifier 122.
Therefore, there is a need for a DAC with improved performance.
Some other conventional circuits are: U.S. Pat. No. 6,369,744; U.S. Pat. No. 6,373,418; U.S. Pat. No. 6,587,060.